Problem Description

In WaterCAD/WaterGEMS V8, what is the effect of toggling the "evaluate as simple
control" check box in the controls window? What is the difference between simple
and logical controls?

Reason

Simple Controls

 1) Used in both steady state and EPS simulations.

 2) Only basic conditions and actions are supported. For example, IF A THEN B. Composite actions cannot be created; for example: IF A OR B THEN C AND D.

 3) Time based controls ignore the operator when evaluated as simple, and always use ‘=’.
- Steady state only uses simple controls, and only if the "use simple controls?"
calculation option is enabled. If it is unchecked, no controls will be considered.

 4) For a particular timestep, the engine goes through iterations to converge. If it converges at an even iteration (for example 8) then simple control conditions are checked. If another simple control condition is met, it iterates again until the hydraulics converge. If the hydraulics converge on an odd timestep, simple controls aren't checked and the hydraulic timestep is complete. The act of only checking on even iterations is a mechanism used to defeat situations where the iterations would go on forever. Basically if the control's action causes another control to trigger and that control causes the first one to trigger, it could go back and forth forever and the model would not be able to converge. This is typically an indication that your controls should be revised. It is important to realize that this can also result in an incorrect solution, for example if another simple control action would have occured, but the solution converged on an iteration where that check is not made. This is a good reason to use logical controls instead, since the logic is different (see below.)


Logical Controls 

 1)  Not used in steady state. (Since the action doesn't apply until the next intermediate
timestep)

 2) Can define more complex controls, with composite conditions/actions (due to the way that they are evaluated) For example: IF A OR B THEN C AND D.

 3)  Instead of determining the exact timestep at which the control would occur (like with simple), it will check every 1/10 of a calculation timestep. If a control condition would occur, the action is applied and an intermediate timestep will be created to reflect this.